The present invention relates to a backlighting device for liquid crystal devices and, more particularly, to a backlighting device promising a uniform illumination over an overall screen for use in display devices such as a liquid crystal display (LCD) device.
Conventional backlighting devices are disclosed in U.S. Pat. No. 4,974,122 entitled "Compact LCD Luminance" and U.S. Pat. No. 4,937,709 entitled "Backlighting Device for a Liquid Crystal Panel." The backlighting device disclosed in the latter patent has been designed to have smaller reflective film patterns in the central portion and larger patterns at the edges of the panel, to circumvent the non-uniformity of luminance which occurs. The backlighting device for an LCD device of the former patent has a light-guiding plate which is thicker at the edges, where lamps are installed than at the median portions.
FIGS. 1 and 2 briefly illustrate a conventional backlighting device for use in LCD devices.
Referring to FIG. 1, the conventional backlighting device comprises a light-diffusing plate 1; a light-guiding plate 3 attached to the light-diffusing plate 1; two column-shaped lamps 2 installed at opposing edges of the light-guiding plate; a reflecting plate 4 installed below the light-guiding plate; and reflecting film patterns 5 between the light-guiding plate and the reflecting plate. Light-guiding plate 3 receives light from the lamps through both edges thereof and guides it, so that light can travel through the light-guiding plate while some of light passes through the light-diffusing plate. Also, reflecting plate 4 is for reflecting the scattered part of the light introduced into light-guiding plate 3 onto light-diffusing plate 1. In particular, reflecting film patterns 5 diffusely reflecting the light onto the light-diffusing plate are located on reflecting plate 4. Here, reflecting film patterns 5 are printed with different sizes symmetrically with respect to horizontal and vertical directions extending from the central portion of light-guiding plate 3, as shown in FIG. 2. In other words, reflecting film patterns 5 become larger gradually, in the longitudinal direction from the central portion of light-guiding plate 3 towards two opposite edges on which lamps 2 are not placed. On the contrary, reflecting film patterns 5 become smaller gradually, in the latitudinal direction from the central portion of light-guiding plate 3 the two opposite edges having lamps 2. Since the luminance of light transmitted from lamps 2 varies according to the distance from the lamps, the reflecting film patterns are printed with the biggest sizes for increasing of luminance by means of the greatest light-reflection, on the portions otherwise having the lowest luminance, i.e., on the portions farthest from the lamps. On the contrary, the reflecting film patterns are printed with the smallest sizes for the lowest luminance on the portions having the highest luminance, i.e., on the portions near to the lamps. Accordingly, uniformity of luminance of the overall LCD panels can be achieved.
However, it is very difficult to print the reflecting film patterns symmetrically parallel or perpendicular to the column-shaped lamps from the central portion of light-guiding plate 3 because the sizes of the reflecting film patterns vary and the high precision required.
Also, if the LCD panel is elongated in the vertical direction parallel to the lamps as in FIG. 2, the sizes of the reflecting film patterns must be unfavorably varied for the preservation of the symmetry between reflecting film patterns lest the luminance of the overall panel should be non-uniform.